1. Field of the Invention
Increasing costs of traditional fuels along with high atmospheric pollution amplify the need for efficient methods of energy capture for cheap, renewable, environmentally friendly power sources. High energy density batteries are conventionally employed for energy storage and release and may be viable energy sources in the future.
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
2. Discussion of the Background
Alkaline earth metals provide attractive anode choices as alternative materials to lithium, for the development of high capacity batteries. Mg is the most attractive and interesting of the alkaline earth metals in this regard because Mg potentially has a volumetric capacity of 3832 mAh cm−3 which is significantly greater than the 2062 mAh cm−3 of Li. Additionally, Mg has a negative reduction potential of −2.356V vs NHE. As the seventh most abundant element in the earth's crust, Mg has a lower resource cost and a lower environmental impact profile (see Aurbach: Nature, Vol 407, pp 724-727, 2000).
Significantly, Mg does not suffer from dendrite formation, which renders Li metal unsafe for commercialization as a high capacity anode material (West: Journal of Electrochemical Communications, Vol 155, pp A806-A811, 2008). Consequently, to avoid this problem, conventional Li ion batteries utilize graphite anodes having a volumetric capacity of only 777 mAh cm−3 (Aurbach: Electrochemical Solid-State Letters, Volume 4, pp A113-A116, 2001).
The electrochemical behavior of a magnesium electrode in a polar aprotic electrolyte solution was reported by Lu et al. in the Journal of Electroanalytical Chemistry (466 (1999) pp 203-217). These authors concluded that the electrochemical behavior of Mg is different from that of Li in polar aprotic electrolyte solutions. Their investigation showed that in contrast to the case of lithium electrodes, surface films which form on the Mg electrode in the aprotic solvents do not transport Mg ions. Therefore, conventional electrolyte systems employed in lithium transport systems are not suitable for a cell having a magnesium anode. Since Mg ion transport is an essential requirement for any electrochemical cell based on a magnesium anode, other electrolyte systems have been investigated.
Gregory et al. (J. Electrochem. Soc., 137 (3), March, 1990, 775-780) reported electrolyte systems of alkylmagnesium halide-organoboron complexes in an ether solvent. Also reported were alkylmagnesium halide solutions to which aluminum halides were added. Mg dissolution and plating at very high current efficiencies, giving bright crystalline Mg deposits were obtained in these systems. However, a suitable cathode material, compatible with the electrolyte system was not reported.
The most commonly used magnesium electrolyte to date is an in-situ generated organometallic material such as phenyl magnesium chloride/aluminum chloride in tetrahydrofuran. However, these electrolyte mixtures are not likely to be of practical commercial utility due to air and moisture sensitivity characteristic of such Grignard-based materials. Moreover, the in-situ generated electrolytes based on reaction of phenyl magnesium chloride/aluminum chloride electrolyte have limited anodic stability, and significantly, such materials are highly nucleophilic and intrinsically strong reducing agents. This chemical reactivity character may be problematic, because to construct an electrochemical cell employing a Grignard type electrolyte, a cathode material which is essentially chemically inert to the Grignard based electrolyte may be required.
Aurbach et al. (NATURE, 407, Oct. 12, 2000,724-726) describes an Mg battery system containing a magnesium organohaloaluminate salt in tetrahydrofuran (THF) or a polyether of the glyme type as electrolyte and a MgxMo3S4 cathode based on a Mo3S4 Chevrel phase host material. A similar cathode material described as having a formula Mg(0-2)MO6S(8-n)Sen was also reported by Aurbach (Advanced Materials, 19, 2007, 4260-4267).
U.S. Pre-Grant Publication No. 2008/0182176 to Aurbach et al. describes an electrochemical cell having a magnesium anode and an intercalation cathode having a modified Chevrel phase. The Chevrel phase compound is represented by the formula Mo6S8-ySey (y is greater than 0 and less than 2) or MxMo6S8 (x is greater than 0 and less than 2). The electrolyte is represented by the formula Mg (AIRxCl4-x)2 and/or (MgR2)x-(AlCl3-nRn)y wherein R is methyl, ethyl, butyl, phenyl and derivatives thereof, n is greater than 0 and lower than 3, x is greater than 0 and lower than 3 and y is greater than 1 and lower than (claim 3) in an ether solvent.
From a practical point of view, with regard to production and use of a commercial battery, electrolyte systems containing agents such as dibutyl magnesium are problematic because of corrosivity, inhalation hazard, flammability and moisture sensitivity.
JP 2004-265675 to Hideyuki et al. describes a test cell constructed with a sulfur containing anode and a negative electrode of magnesium metal. Magnesium bis(trifluoromethylsulfonyl)imide in γ-butyrolactone is employed as an electrolyte system.
However, Mg batteries cannot utilize commercially available aprotic ionic salts such as magnesium bis(trifluoromethanesulfonyl)-imide and magnesium perchlorate because of the reported formation of a solid electrolyte interface (SEI) film impermeable to Mg ions which prohibits deposition/dissolution (Feng, Z: Surface Coating Technologies, Vol 201, pp 3783-3787, 2006).
The ideal Mg battery electrolyte would be electrochemically and chemically stable in battery operation conditions, would have high ionic conductivity but be an electronic insulator, contain ions of the Mg anode material and have low melting and high boiling points (Xu, K: Chemical Reviews, Vol 104, pp 4303-4417, 2004). This electrolyte would also need to be inert to battery components such as anode, cathode or current collector. Aurbach et al. was the first to report a family of magnesium organohaloaluminate electrolytes which are compatible with magnesium anodes (Aurbach: Nature, Vol 407, pp 724-727, 2000, Aurbach: 6,316,141). They are generated in situ from the reaction between a Lewis acid and a Lewis base such as phenylmagnesiumchloride and aluminum trichloride (AlCl3), respectively, in tetrahydrofuran. Muldoon et al. was the first to report the crystallization of the electrochemically active species (Mg2(μ-Cl)3.6THF)(HMDSnAlCl4-n) (n=1, 2), formed from the reaction of hexamethyldisilizide (HMDSMgCl) and AlCl3 in tetrahydrofuran. (Muldoon: Nature Communications 10.1038/ncomms1435, 2011; U.S. application Ser. No. 12/758,343, filed Apr. 12, 2010; U.S. application Ser. No. 12/768,017, filed Apr. 27, 2010). However, while these electrolytes have voltage stabilities above 3V vs Mg on a Pt working electrode, their voltage stability drops to 2.3V vs Mg on stainless steel working electrode (FIG. 1).
The stainless steel compatibility problem may be overcome by using carbon cloth current collectors (Muldoon: Nature Communications, 10.1038/ncomms1435). Doe et al. (U.S. 2011/0159381, filed Mar. 8, 2011) reports the dependence of the oxidative stability of Mg organohaloaluminates electrolytes on a variety of metals and describes a magnesium battery electrode assembly having a current collector containing a carbonaceous material to avoid this problem.
However, improving the voltage stability of magnesium electrolytes on stainless steel is crucial because stainless steel is a widely used current collector and a major component in a variety of batteries such as coin cells. Current state of the art magnesium organohaloaluminate electrolytes limit the usage of Mg battery coin cells to operating under 2.3V vs Mg.